1. Field of the Invention
The present invention relates to an inorganic material used as a negative electrode active material for a lithium ion secondary battery, and a method for producing the same.
2. Description of the Related Art
In recent years, different types of lithium ion secondary batteries have been developed. Carbon materials have been conventionally used as a negative electrode active material for a lithium ion secondary battery. However, lithium titanium complex oxide materials have recently been developed and attracting public attention. For example, a lithium ion secondary battery using LiCoO2 as the positive electrode active material and Li4Ti5O12 as the negative electrode active material has already been put to practical use.
Li4Ti5O12 is a material having a spinel crystalline structure and is capable of repeatedly absorb and release Li, and therefore Li4Ti5O12 can be used as an active material for a lithium ion secondary battery. Li4Ti5O12 absorbs and releases Li at a potential of about 1.5 V with respect to the standard oxidation-reduction potential of lithium (Li/Li+). Therefore, it is believed that where Li4Ti5O12 is used as a negative electrode active material in a lithium ion secondary battery, lithium metal is unlikely to deposit on the negative electrode even if there occurs a reaction overvoltage resulting from rapid charging, or the like, thus realizing a lithium ion secondary battery with a high degree of safety. It also offers desirable cycle characteristics because there is little lattice expansion from charging/discharging.
However, Li4Ti5O12 has a low electronic conductivity. Therefore, where Li4Ti5O12 is used in a lithium secondary battery, a problem remains with the output characteristics. In order to solve this problem, attempts have been made to add a different element to Li4Ti5O12 to alter the property thereof. For example, Japanese Laid-Open Patent Publication No. 2000-277116 discloses a material in which a portion of Ti element of Li4Ti5O12 is substituted with a different element selected from V element, Nb element, Mo element and P element for the purpose of improving the output characteristics by increasing the electronic conductivity. Japanese Laid-Open Patent Publication No. 2000-277116 reports that the electronic conductivity is increased and the discharge characteristics at high loads are improved by substituting a portion of Ti element with a different element selected from V element, Nb element, Mo element and P element. However, the discharge capacity at high loads is 83% or less that at low loads, and it cannot be said that the discharge characteristics are sufficient.
Japanese Laid-Open Patent Publication No. 2000-156229 discloses a material in which a portion of Ti element of Li4Ti5O12 is substituted with a transition metal element other than Ti element. While Japanese Laid-Open Patent Publication No. 2000-156229 reports that the storage stability improves by substituting a portion of Ti element with various transition metal elements, it provides no specific reference to the output characteristics or the electrode capacity density. Also, the actual syntheses, the resulting production of intended lithium titanium complex oxides, and the property of the produced materials are only reported for cases in which the substituting element is B element, Co element or Zn element.
Japanese Laid-Open Patent Publication Nos. 2005-228706 and 2004-319105 disclose that Li4Ti5O12 or Li4Ti4.90Mn0.10O4 can be used also as a coating material of a positive electrode active material. However, Japanese Laid-Open Patent Publication Nos. 2005-228706 and 2004-319105 provide no specific reference to the applicability or characteristics thereof as a negative electrode active material.